RFR: 8260165: CSSFilterTest.testCSSFilterRendering system test fails

[Arun Joseph]

Issue: Initial layout delay was removed and layout() is called from layoutTimer instead of WebPage::prePaint().

Fix: Re-introduce the initial layout delay.

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Commit messages:
 - Add comment
 - 8260165: CSSFilterTest.testCSSFilterRendering system test fails

Changes: https://git.openjdk.java.net/jfx/pull/408/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=408&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260165
  Stats: 8 lines in 2 files changed: 6 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/408.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/408/head:pull/408

PR: https://git.openjdk.java.net/jfx/pull/408